This invention relates generally to a felt for use in a papermaking machine, and more particularly to such a felt constructed as having a surface layer of coarse fibers and an underlayer of relatively fine fibers for minimizing paper sheet rewetting as the water within the felt migrates away from the sheet side to the finer underlayer.
Rewetting is a phenomenon in which water pressed from the sheet at mid-nip re-enters the sheet on the exit side of the nip. The theory of rewetting has been discussed by P. B. Wahlstrom, Pulp and Paper Magazine Canada, 70, No. 19: 76 (1969). Wahlstrom pointed out that capillary transfer in the felt/sheet interface appears to be the controlling mechanism in rewetting. He also emphasized the importance of felt design in minimizing rewetting.
In the current decade, designers of needled felts have sought to minimize rewetting by stratification of batt using a fine surface layer on the sheet side to create the desired capillary forces. Such a felt design is disclosed in U.S. Pat. No. 3,928,699. It has been found, however, that inverse batt stratification, i.e., a surface layer of coarse fibers and an underlayer of finer fibers, more effectively minimizes sheet rewetting.
It is generally known that capillary attraction or repulsion forces are the resultant of adhesion, cohesion and surface tension in liquids which are in contact with solids as in a capillary tube. When the cohesive force is greater the surface of the liquids tends to rise in the tube, and when the adhesion force is greater the surface tends to be depressed in the tube. This can be directly related to the denier (coarseness) of the fibers used in batt material on wet felt. Accordingly, this would mean that the finer the fibers, the more efficient the water absorbtion and vice versa.